The international community is devoting increasing resources to innovation in medium and high power static conversion, up to medium voltage levels, in response to the potential of new applications such as energy generation from renewables and the increasing requirement for improved conversion and compatibility in conventional applications such as the supply of electric motors in existing installations.
The common factor in all these activities can be summarized as the application of the regulating properties of PWM (Pulse Width Modulation), made possible by forced switching, replacing the more limited properties of natural switching of the controlled diode type, while providing a radical solution to the problems of compatibility and efficiency posed by the prior art PWM structures.
Specifically, the prior art of ‘two-level’ forced switching is represented by simple circuit structures in which the electrical quantities are generated and regulated by pulses at two values or levels which delimit the range of regulation. Forced switching between these extreme values or levels produces a quantity of dissipated energy which, at the high switching frequency required by PWM methods, has a dominant effect in energy terms on the overall efficiency.
The reduction of the switching times, and therefore of energy losses, is not achievable because of the dynamic limits of the semiconductors or physical layout limitations, leading to the emission of electromagnetic interference which makes a converter incompatible unless it is provided with complex partially dissipating filters to ensure electromagnetic compatibility, which would make the converter bulky.
The compromise between switching performance and electromagnetic compatibility, which is fundamental for the widespread adoption of industrial electrical drives, is also based on the opportunity of using new electrical machines, or conventional machines redesigned on an ad hoc basis, which can withstand the dynamic stresses of switching in order to exploit the inductive reactive properties as a functional part of the conversion.
In this context, it is evident that generation applications in an electrical network cannot be equated to industrial drive applications. For generation applications, it is necessary to attenuate the discontinuities of PWM switching before they reach the converted electrical circuit, by providing filters (or reactive dipole filters) which have a major effect on the volume, weight, efficiency and cost of a complete PWM converter.